This invention relates to a matrix display device comprising sets of row and column address conductors, a row and column array of picture elements operable to produce a display, each of which comprises an electro-optic display element connected in series with a two terminal non-linear device exhibiting a threshold characteristic between a row conductor and a column conductor, and picture element drive means connected to the sets of address conductors for applying drive voltages to the picture elements comprising a scanning signal drive circuit for applying selection signals to the conductors of one set and a data signal drive circuit for applying data signals to the conductors of the other set. The invention relates also to a method of operating such a display device.
Display devices of this kind are suitable for displaying alpha-numeric or video information using passive electro-optical display media such as liquid crystal material, electrophoretic suspensions or electrochromic materials. Examples of such display devices, using liquid crystal material, are described in GB-A-2129182, EP-A-0185995, and GB-A-2147135. The two terminal non-linear devices can be of various forms, such as diode rings, back to back diodes, MIMs, etc., which are bidirectional. The polarity of the drive voltages applied to the picture elements can then conveniently be inverted periodically, typically in successive field periods, in order to prevent degradation of the electro-optic display material and improve display quality. The picture elements are addressed by sequentially applying a selection voltage signal to each one of the first set of address conductors, usually the row conductors, and data, for example video, signals to the other set of address conductors to set the display elements to a desired display condition which is maintained until they are again selected.
For acceptable quality of display it is important that the non-linear devices of the matrix array demonstrate substantially similar threshold and I-V characteristics in operation so that the same drive voltages applied to any picture element in the array produce substantially identical visual results, for example in the case of a liquid crystal display device, as regards picture element transmission levels. Differences in the threshold or turn-on point of the non-linear devices can appear directly across the electro-optical material producing different display effects from picture elements addressed with the same drive voltages.
Serious problems can arise if the threshold level of the non-linear devices changes over a period of time, for example through ageing effects. The consequential change in display element voltages not only leads to inferior display quality but, depending on the drive scheme employed, can cause an image storage problem and also degradation of the LC material.
In the aforementioned GB-A-2129182 a drive scheme is described which involves a four level row drive in which the scanning signal applied to a row conductor consist of first, selection, voltage level for a selection interval of fixed duration followed by a second, hold, voltage level of less value but of the same polarity as the selection level and which is maintained for at least a major portion of the time which elapses until the row conductor is next addressed with the selection voltage level. The polarity of the selection and hold levels is inverted for successive field periods. It is said that by using this method non-linear devices having a comparatively low threshold voltage would be sufficient allowing relatively low drive voltages. There is also described briefly in this specification a reference voltage setting circuit which is used to adjust the selection and hold voltages applied to the picture elements in accordance with changes in the threshold voltage level of a non-linear element caused by variations in operating temperatures in use of the display device. This circuit uses a reference non-linear element, namely a diode element, one side of which is connected to ground, and operates to compare the threshold voltage of the reference element with reference potentials comprising a predetermined threshold voltage level. This is achieved by sensing the voltage across the reference element.